In the wake of the discovery of oxide superconductors, technological development of such oxide superconductors has recently been progressing at a rapid pace. It is known that the oxygen content of an oxide superconductor generally has great influences on its superconducting properties. In the production of an oxide superconductor, however, an extreme difficulty is encountered in securing a predetermined oxygen content in the resulting superconductor.
For example, a Ba.sub.2 YCu.sub.3 O.sub.x superconductor, where x (which may be zero) represents the quantitative proportion of oxygen in the composition of the superconductor, is produced by mixing barium carbonate (BaCO.sub.3) with yttrium oxide (Y.sub.2 O.sub.3) and copper oxide (CuO) at a predetermined ratio, compression-molding the resulting mixture into a pellet, and heating the pellet in an oxygen-filled furnace under atmospheric pressure at 930.degree. C.
As is well known, the pellet thus treated is of tetragonal crystal structure at a temperature of 930.degree. C. When this oxide pellet is dipped quickly into liquid nitrogen to be quenched from 930.degree. C., it still maintains the tetragonal crystal structure. However, this pellet cannot show superconductivity because it has a low oxygen content (see, for example, H. Ihara, et al. Physica C 153-155 (1988) 948-949). In the following description, oxide substances incapable of showing superconductivity because of their low oxygen contents will be referred to as "tetragonal structure substances".
In order to increase the oxygen content of the pellet to a suitable value to provide the best superconducting substance, the pellet must usually be slowly cooled from the above-mentioned temperature of 930.degree. C. over one day and night through furnace cooling. The pellet cooled in this manner is of orthorhombic crystal structure and shows superconductivity at temperatures of around 90.degree. K. or lower. In the following description, orthorhombic oxide superconducting substances showing superconductivity at temperatures of around 90.degree. K. or lower will be referred to as "orthorhombic structure substances".
Thus, according to the foregoing conventional method, a pellet must be slowly cooled from 930.degree. C. over one day and night through furnace cooling in order to secure superior superconducting properties. Furnace cooling over such a long period of time is not commercially feasible to industrial producing of superconductors.
B. G. Bagley et al. reported in their paper [Appl. Phys. Lett. 51 (1987) p. 622-p. 624] that a tetragonal structure substance incapable of showing superconductivity was subjected to a radiofrequency discharge treatment in an atmosphere of oxygen to be changed into an orthorhombic structure substance capable of showing superconductivity. In this case, however, plasma oxidation treatment over 285 hours is necessary for securing a 90-K class superconductor which exhibits superconductivity at temperatures of around 90.degree. K. or lower. Plasma oxidation treatment over such a long period of time is industrially impractical as well.
S. Minomo et al reported in their paper (Jpn. J Appl. Phys. 27 (1988) p. L411-L413) that a superconducting substance of 60-K class which shows superconductivity at temperatures of around 60-K or lower was heated to 400.degree. C. and subjected to an ECR (electron cyclotron resonance) discharge treatment at 400.degree. C. for 30 minutes, in order to be changed into an improved superconducting substance having a 90-K class of superconductivity. However, this process is concerned only with discharge and heating treatments of a substance, already capable of showing superconductivity in itself, to improve the superconducting properties thereof. Hence, the Minomo method is different from a treatment of changing the crystal structure of a tetragonal substance into a superconducting crystal structure.
Tetragonal structured oxide substances show semiconductor-like characteristics of electric resistance with the temperature shown in FIG. 1(a), while orthorhombic structured oxide superconducting substances show the characteristics of electric resistance with the temperature shown in FIG. 1(b), which corresponds to the characteristics of superconductors. In FIGS. 1(a) and 1(b), the abscissa represents the temperature (K) on an arbitrary scale, while the ordinate represents the electric resistance (.OMEGA.) on an arbitrary scale. The superconductive transition temperature, or critical temperature T.sub.c, is a temperature at and below which superconductivity is shown. Therefore, in the case of the superconducting substance reported by S. Minomo et al., it is important to note that "the crystal structure of the substance was not changed." In order to change a tetragonal structure substance into an orthorhombic structure substance having superconducting properties, the former must usually be treated in an atmosphere of oxygen at a high temperature (at 300.degree. C., no change thereof occurs even when it is treated for 2 hours). Further, it has heretofore been impossible to change a tetragonal structure substance quenched from a high temperature into an orthorhombic structure substance having superconducting properties within a short period of time.
Etatsu et al. reported in 1988 35th Oyobutsurigaku -Kanren Koenkai Extended Abstracts (The 35th Spring Meeting, 1988) The Japan Society of Applied Physics and Related Societies 29a-X-5 that a tetragonal structure substance was locally changed into an orthorhombic structure substance through oxygen ion implantation and laser annealing. However, this procedure is an extremely local treatment which is, therefore, inapplicable to large scale production of broad-area superconductors.
As described above, in order to change a tetragonal structure substance into an orthorhombic structure substance having good superconducting properties, the tetragonal structure substance must be treated in an atmosphere of oxygen in a furnace at a temperature higher than at least 400.degree. C. for a long period of time, or must be treated in an oxygen plasma for a period of time longer than at least 200 hours. Thus, all the foregoing conventional methods involving an annealing-in-oxygen (O.sub.2) treatment under atmospheric pressure requiring a long treatment time posed a problem of inadaptability to large scale production of superconductors.
Further, through the aforementioned ECR discharge treatment method is capable of improving the superconducting properties of an orthorhombic structure substance, it is incapable of changing a tetragonal structure substance into an orthorhombic structure substance.